1. Field of the Invention
The present invention relates to a system and method for expanding a laser beam.
2. Background Art
In many applications, the laser beam emitted by a laser needs to be expanded. Expansion of a laser beam is necessary in microlithography because a desired illumination system field is typically much bigger than the cross-sectional area of a beam emitted directly from the laser (also called a laser emission footprint). For example, one type of excimer laser beam has a laser emission footprint that is a generally rectangular area of about 5 mm×15 mm. On the other hand, an illumination system in a lithography tool may require an illumination field which is much larger on the order of 10 mm by 120 mm. This requires significant expansion of the laser beam.
One conventional approach for laser beam expansion involves the use of refractive elements, such as, a lens or prism structure. Unfortunately, expansion with such refractive elements increases the spatial coherence cell sizes of the laser and creates a speckle problem in the final image. To deal with the increasing spatial coherence cell sizes and bandwidth of the laser, a structure consisting of multiple lenses of different length has been used to expand the laser beam. Such a structure, however, is expensive and impractical for expanding a laser beam of reduced bandwidth to a degree involving hundreds or thousands of spatial cells.
Therefore, what is needed is an improved system and method for expanding a laser beam of reduced bandwidth.